[analyzer] Fix crash for array-delete of UnknownVal values.

We now skip the destruction of array elements for `delete[] p`,
if the value of `p` is UnknownVal and does not have corresponding region.
This eliminate the crash in `getDynamicElementCount` on that
region and matches the behavior for deleting the array of
non-constant range.

Reviewed By: isuckatcs

Differential Revision: https://reviews.llvm.org/D136671